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We independently manipulated mixing intensity (strong artificial mixing vs. background turbulence) and water‐column depth (2 m, 4 m, 8 m, and 12 m) in order to explore their separate and combined effects in a field enclosure experiment. To accentuate the vertical light gradient, enclosures had black walls, resulting in a euphotic depth of only 3.7 m. All enclosures were placed in a well‐mixed water bath to equalize temperature across treatments. Phytoplankton responded to an initial phosphorus pulse with a transient increase in biomass, which was highest in the shallowest, least light‐limited water columns where dissolved mineral phosphorus subsequently became strongly limiting. As a consequence, the depth‐averaged mineral phosphorus concentration increased and the seston carbon (C) : phosphorous (P) ratio decreased with increasing water‐column depth. Low turbulence enclosures became quickly dominated by motile taxa (flagellates) in the upper water column, whereas mixed enclosures became gradually dominated by pennate diatoms, which resulted in higher average sedimentation rates in the mixed enclosures over the 35‐d experimental period. Low turbulence enclosures showed pronounced vertical structure in water columns >4 m, where diversity was higher than in mixed enclosures, suggesting vertical niche partitioning. This interpretation is supported by a primary production assay, where phytoplankton originating from different water depths in low‐turbulence treatments had the relatively highest primary productivity when incubated at their respective depths of origin.
Transient dynamics of pelagic producer-grazer systems in a gradient of nutrients and mixing depths
(2008)
Phytoplankton–grazer dynamics are often characterized by long transients relative to the length of the growing season. Using a phytoplankton–grazer model parameterized for Daphnia pulex with either flexible or fixed algal carbon : nutrient stoichiometry, we explored how nutrient and light supply (the latter by varying depth of the mixed water column) affect the transient dynamics of the system starting from low densities. The system goes through an initial oscillation across nearly the entire light–nutrient supply space. With flexible (but not with fixed) algal stoichiometry, duration of the initial algal peak, timing and duration of the subsequent grazer peak, and timing of the algal minimum are consistently accelerated by nutrient enrichment but decelerated by light enrichment (decreasing mixing depth) over the range of intermediate to shallow mixing depths. These contrasting effects of nutrient vs. light enrichment are consequences of their opposing influences on food quality (algal nutrient content): algal productivity and food quality are positively related along a nutrient gradient but inversely related along a light gradient. Light enrichment therefore slows down grazer growth relative to algal growth, decelerating oscillatory dynamics; nutrient enrichment has opposite effects. We manipulated nutrient supply and mixing depth in a field enclosure experiment. The experimental results were qualitatively much more consistent with the flexible than with the fixed stoichiometry model. Nutrient enrichment increased Daphnia peak biomass, decreased algal minimum biomass, decreased the seston C:P ratio, and accelerated transient oscillatory dynamics. Light enrichment (decreasing mixing depth) produced the opposite patterns, except that Daphnia peak biomass increased monotonously with light enrichment, too. Thus, while the model predicts the possibility of the “paradox of energy enrichment” (a decrease in grazer biomass with light enrichment) at high light and low nutrient supply, this phenomenon did not occur in our experiment.